For example, in a power conversion apparatus in which switching semiconductor devices are controlled to be ON/OFF to supply a sinusoidal current for variable-speed driving a load of a variable-speed pumping generator and the like, when the output frequency of the power conversion apparatus (for example, a three-level converter) is a relatively low frequency, the time period during which the sinusoidal current is positively or negatively conducted is prolonged. This lengthens the time period during which the switching semiconductor devices are conducted, and the junction temperature of the switching semiconductor devices continues rising during this time period, with the result that overheating may finally cause breakage.
The following is proposed as a technique for protecting a switching semiconductor device from overheating as described above.
Japanese Patent Laying-Open No. 2008-178188 (PTD 1) aims to provide a power conversion apparatus that, when the output frequency becomes relatively low, operates for an increased duration under a certain load condition required by a system and is also capable of continuously operating even under the load condition different from that required by the system. The invention disclosed in Japanese Patent Laying-Open No. 2008-178188 (PTD 1) provides a power conversion apparatus including a power converter that has a plurality of switching semiconductor devices and controls these switching semiconductor devices to be ON or OFF to supply a sinusoidal current to an alternating-current (AC) electric motor. The power conversion apparatus is configured to include frequency correction means. When the output frequency of the power converter becomes a relatively low frequency that is equal to or less than a prescribed value, the frequency correction means increases the output frequency in proportion to the equivalent of the heat load of the switching devices calculated by operation.